Do I have to fit an appliance exactly to the manufacturer’s instructions?

Yes. All appliances must be fitted exactly to the manufacturer’s instructions (see Clause 6.5).
This means you cannot change a flue, or reduce the specified gas pipe size, and the inlet pipe must be correct as specified (i.e. we can only use a hose if the appliance is “specified” to be used with a hose).
There are also other requirements within the standard: Once installed the appliance must be accessible for service (see Clause 2.9.2 and 6.5.3) and the test point must be accessible for testing gas pressure (see Clause 5.1.7).
When installed and commissioned the test pressure must be correct (see Appendix J).